The present invention is directed to metal particles used for forming coating compositions. Such particles are used, for example, to provide metallic coatings for automotive use and in general maintenance, industrial and roof coating systems. The metal particles are typically in the form of flakes, and are provided in the form of a paste that is mixed with a suitable vehicle or carrier to form a coating.
In the past, organic solvent-based vehicles have been used for coating compositions, with good results. However, the use of such organic-based coating compositions results in the release of significant amounts of volatile organic material into the atmosphere, which has been determined to be damaging to the environment, and increasingly strict regulations against organic emissions have been implemented. One way for coating compositions to comply with the new regulations is to convert from organic-based vehicles for the compositions to aqueous-based vehicles, since in such cases water is released to the atmosphere instead of volatile organic material.
In the case of metallic coating compositions, the use of aqueous vehicles is problematic, since water can react with the metal particles to generate hydrogen gas. This is particularly true for zinc and aluminum particles. The reaction of aluminum particles with water is depicted below as an example. EQU 2Al+6H.sub.2 O.fwdarw.2Al(OH).sub.3 +3H.sub.2
The amount of gas generated can produce a safety hazard, resulting in high pressures in composition containers. In addition, the water reaction significantly reduces the aesthetic value of the metal particles.
A number of techniques have been proposed for combating the problem of water reaction with metal particles in aqueous coating systems. One example is the passivation of metal particles with organic phosphate (U.S. Pat. No. 4,565,716). Another is the use of hexavalent chromium or pentavalent vanadium compounds (U.S. Pat. No. 4,693,754). Other techniques include the use of organic phosphites (U.S. Pat. No. 4,808,231) and the use of nitroparaffin solvents, and the use of nitroparaffin solvents in combination with organic phosphate, organic phosphite or pentavalent vanadium compound (U.S. Pat. No. 5,215,579). The use of heteropolyanions and phosphosilicate pigments as treating agents, alone and in combination, also has been contemplated (U.S. Pat. Nos. 5,296,032, 5,348,579 and 5,356,469). The disclosures of each of the patents mentioned above are incorporated herein by reference. While some degree of success has been achieved with these treatments, still further improvements in providing resistance to attack from aqueous carriers for metal particles having desired properties for coating compositions have been desired.